Ionizing radiation energy deposited in a scintillator material is converted into light. This light can then be measured by photo-sensitive detectors. Generally, incident penetrating radiation includes high-energy particles and ionizing radiation such as x-rays, gamma rays, alpha and beta particles, and/or fast and thermal neutrons. Plastics can be used as a scintillator material. Plastic scintillators can be used, for example, to detect the presence of ionizing radiation from illegal transport of radioactive and fissile material, in monitoring and safeguarding nuclear stockpiles, in service of nuclear nonproliferation, in the operation of nuclear research and power reactors, in monitoring the use of medical and industrial isotopes, and in high energy, cosmic, and nuclear basic research.
In all of these applications, there has been a long term need for increasing the efficiency and discrimination of detecting neutrons in the presence of background gamma rays. The detection of neutrons is important because they are strongly indicative of the presence of fissile material, such as plutonium and enriched uranium.
Gas proportional tubes have been used extensively to detect thermal neutrons with good discrimination against gamma rays. However, these tubes suffer from some disadvantages, including inability to detect fast neutrons, sensitivity to mechanical vibration and shock, poor timing resolution, and high cost per unit area. Some of the above disadvantages are also present with respect to another neutron detection technology, namely, lithium loaded zinc sulphide screens read out by plastic optical wavelength shifting fibers.
Organic liquid scintillators have been employed to detect fast neutrons, partly because their high hydrogen content allows neutron detection via proton recoil (G. F. Knoll, “Radiation Detection and Measurement”, J. Wiley and Sons, 1998). For these detectors, the discrimination of the neutrons over gamma rays is achieved with the use of pulse shape discrimination (PSD). With PSD, the gamma ray and neutron scintillation pulses are distinguished by the unique temporal signal characteristics of neutrons. However, the discrimination against gamma-rays is less than desired in a high background of gamma-rays. A recent review of progress in PSD in liquid scintillators has demonstrated that the technique has become very powerful in commercially available liquid scintillators; see Mark Flaska and Sara A. Pozzi, Nuclear Instruments and Methods in Physics Research, Vol. 599, Issue 2-3, 221-225 (2009). In particular, they have shown that scintillation pulses produced by nuclear fragments resulting from thermal neutron capture by 10B have shapes that can be distinguished from both neutron scattering pulses and from gamma ray interactions. This permits the detection of a neutron scatter followed by the neutron being captured in the same detector. This so called “capture-gated” detector provides measurement of the energy spectrum of fast neutrons followed by neutron capture identification and excellent gamma rejection. At present, a few liquid organic scintillators featuring the PSD enabling property are commercially available for neutron detection (e.g., from Bicron Corp. and Eljen Technology). However, there are safety concerns in using large volumes of toxic, flammable, aromatic liquids at port and border locations having high commercial activity and/or utilizing large moving trucks. For these reasons liquid scintillators have not been deployed at these locations.
Among solid organic materials, single crystals of Stilbene and some other crystalline dyes have been found to exhibit comparably effective PSD as liquid scintillators (Hull et al. IEEE Transactions on Nuclear Science, Vol. 56, No. 3., 899-903.). The difficulty of growing large crystals makes it unlikely they will be used in anything other than hand-held detectors.
There have been advancements in the last decade in the development of ultrafast waveform digitizers (e.g., from Agilent Technologies), which have enabled commercial detectors featuring PSD.
In some of the above applications, the shipping container hiding the fissile material is exposed to an external beam of gamma rays or neutrons. Known as active interrogation, this process creates a relatively high radiation exposure to the container and its surroundings. In this case, the scintillator that offers PSD is required to operate in a high radiation environment with very high gamma discrimination.
In summary, there is a continuing need to have large area, cost effective, robust plastic scintillator material, method, and apparatus, offering fast and thermal neutron detection by PSD, excellent gamma discrimination, good radiation resistance, and good timing information.